To calculate net carbs, subtract fiber and sugar alcohols from the total carbohydrates listed on a nutrition label. The basic formula is: Net Carbs = Total Carbohydrates − Fiber − Sugar Alcohols. Most people following a ketogenic diet aim for 20 to 50 grams of net carbs per day to stay in ketosis, so getting this math right matters.
The concept behind net carbs is straightforward: your body can’t fully digest fiber or most sugar alcohols, so they don’t raise blood sugar the way starches and sugars do. By subtracting them, you get a closer estimate of the carbohydrates that actually affect your metabolism.
The Basic Formula
For whole foods like vegetables, nuts, and fruits, the calculation is simple. Look at the nutrition label (or look up the food in a database) and subtract the dietary fiber from total carbohydrates. A cup of broccoli with 6 grams of total carbs and 2.4 grams of fiber, for example, has about 3.6 net carbs.
For packaged foods that contain sugar alcohols, you subtract those too. If a protein bar lists 24 grams of total carbohydrates, 3 grams of fiber, and 10 grams of sugar alcohols, your net carb count would be 24 − 3 − 10 = 11 net carbs. That’s the version most keto resources teach, and it works well as a starting point. But certain sugar alcohols deserve a closer look.
Not All Sugar Alcohols Are Equal
Sugar alcohols break down slowly in the gut, and your body only absorbs part of their carbohydrates. That keeps blood sugar and insulin from spiking the way regular sugar does. You can spot them on ingredient lists by the “-ol” at the end of their names: erythritol, xylitol, sorbitol, maltitol, mannitol, and others.
Erythritol has virtually zero impact on blood sugar. You can subtract 100% of its carbohydrates from the total. Xylitol and sorbitol have a mild effect, and many keto practitioners subtract them fully as well, though some prefer a more conservative approach.
Maltitol is the one to watch. It raises blood sugar significantly more than other sugar alcohols. The common recommendation is to subtract only half of maltitol’s grams from total carbohydrates. So if a label shows 20 grams of total carbs and 10 grams of sugar alcohol from maltitol, you’d subtract 5 grams (half of 10) rather than the full 10, giving you 15 net carbs instead of 10. Many “sugar-free” candy bars and chocolates use maltitol, which is why they can still kick people out of ketosis despite looking low-carb on the label.
The Fiber Subtraction Has Limits Too
All fiber listed on a U.S. nutrition label can technically be subtracted, since the FDA’s definition includes both naturally occurring plant fibers and approved isolated or synthetic fibers like inulin, psyllium husk, beta-glucan, cellulose, and resistant starch. These don’t raise blood sugar in a meaningful way.
Some keto practitioners use a more conservative rule: when a food has more than 5 grams of total fiber per serving, subtract only half the fiber from total carbohydrates instead of the full amount. This accounts for the possibility that some labeled fibers, particularly added functional fibers in processed foods, may still contribute a small number of usable calories. For whole foods like leafy greens or avocados, subtracting all the fiber is perfectly reasonable.
What About Allulose?
Allulose is a rare sugar that shows up in more and more keto-friendly products. It tastes like sugar but contributes only about 0.4 calories per gram (compared to 4 calories per gram for regular sugar). The FDA has advised manufacturers that allulose can be excluded from “Total Sugars” and “Added Sugars” on nutrition labels.
Here’s where it gets tricky: some manufacturers still include allulose in the total carbohydrate count because the FDA hasn’t finalized a formal rule excluding it. If you see allulose listed in the ingredients and the total carb count seems high for what should be a low-carb product, check whether allulose is inflating that number. You can subtract allulose from total carbohydrates when calculating net carbs, since it has almost no effect on blood sugar.
Reading U.S. vs. International Labels
This entire calculation assumes you’re reading a U.S. nutrition label, where total carbohydrates include fiber. In the U.K. and most of Europe, nutrition labels list “carbohydrates” and “fibre” separately, and the carbohydrate number typically already excludes fiber. If you’re reading a European label and subtract fiber again, you’ll undercount your carbs.
A quick way to check: if the label lists fiber as a sub-item indented under carbohydrates (as U.S. labels do), subtract it. If fiber appears as its own separate line at the same level as carbohydrates, the carb number likely already excludes it and no subtraction is needed.
Putting It Into Practice
For whole, unprocessed foods, the math is clean. Subtract all the fiber from total carbs and you have your net carbs. An avocado, a handful of almonds, a serving of cauliflower: these are straightforward.
For packaged keto products, use this decision tree:
- Erythritol or allulose: Subtract 100% from total carbs.
- Xylitol, sorbitol, or most other sugar alcohols: Subtract 100%, or subtract half if you want to be conservative.
- Maltitol: Subtract only 50% from total carbs.
- Fiber from whole food sources: Subtract 100%.
- Added functional fibers (in bars, shakes, and processed snacks): Consider subtracting only 50% if the fiber content is above 5 grams per serving.
As a practical example, imagine a keto chocolate bar with this label: 22 grams total carbohydrates, 8 grams fiber (from chicory root), 10 grams erythritol, and 2 grams maltitol. You’d subtract the full 8 grams of fiber, the full 10 grams of erythritol, and half the maltitol (1 gram). That gives you 22 − 8 − 10 − 1 = 3 net carbs.
How Many Net Carbs to Stay in Ketosis
The ketogenic diet typically keeps total carbohydrate intake below 50 grams per day, with many people targeting 20 grams of net carbs as a starting point. At 20 grams, almost everyone enters ketosis. At 50 grams, many people can maintain it, especially if they’re physically active. The right number for you depends on your metabolism, activity level, and how long you’ve been eating this way.
Tracking net carbs rather than total carbs gives you more room to eat fiber-rich vegetables, nuts, and seeds without blowing your daily budget. A large salad with spinach, avocado, and a handful of pecans might look carb-heavy at first glance, but once you subtract the fiber, it fits comfortably into a keto day. That flexibility is the entire reason the net carb concept exists.